Certain optical media, including at least some silica-based optical fibers, can be modified by exposure to electromagnetic radiation in an appropriate spectral range. (Ultraviolet radiation is typically used.) That is, exposure of a photosensitive optical fiber (or other optical medium) to appropriate radiation may cause the refractive index to change in the exposed portion of the medium. A periodic pattern can be imposed on the impinging radiation by, e.g., superimposing a pair of beams of substantially monochromatic radiation to create an interference pattern. When such a patterned radiation field impinges on an optical fiber or other optical waveguide having a core of the appropriate photosensitivity, a corresponding pattern is imposed on the core in the form of periodic (or quasiperiodic) fluctuations in the core refractive index. Such a pattern, which is referred herein to as a "Bragg grating," can behave as a spectrally selective reflector for electromagnetic radiation.
A technique for creating these Bragg gratings is described in U.S. Pat. No. 4,725,110, issued to W. H. Glenn, et al. on Feb. 16, 1988, and U.S. Pat. No. 4,807,950, issued to W. H. Glenn, et al. on Feb. 28, 1989. An optical fiber laser having a DBR-terminated cavity is described in G. A. Ball and W. W. Morey, "Continuously tunable single-mode erbium fiber laser," Optics Lett. 17 (1992) 420-422.
Bragg gratings are useful, inter alia, as spectral filters. An optical filter which comprises a Bragg grating formed in an optical fiber is described in U.S. Pat. No. 5,007,705, issued to W. W. Morey, et al. on Apr. 16, 1991.
Practitioners in the art have reported that Bragg gratings formed in this manner are anisotropic. That is, an optical signal of a given vacuum wavelength has an effective wavelength within the Bragg grating that is polarization-dependent. As a consequence, the wavelength of maximum reflectivity of the Bragg grating tends to shift as the polarization of a linearly polarized input signal is rotated. The anisotropy of Bragg gratings in optical fibers has been reported, for example, by G. Meltz and W. W. Morey, "Bragg Grating Formation and Germanosilicate Fiber Photosensitivity," SPIE International Workshop on Photoinduced Self-Organization in Optical Fiber, Quebec City, Canada (May 10-11, 1991), SPIE Vol. 1516. Such anisotropy has also been reported by K. O. Hill, et at., "Birefringent Photosensitivity in Monomode Optical Fibre: Application to External Writing of Rocking Filters," Electr. Lett. 27 (1991) 1548-1550.
However, there are many applications in which signal polarizations are random, and it is consequently desirable for signal-processing components to perform in a manner which is independent of polarization. Practitioners in the art have hitherto failed to provide a technique for forming Bragg gratings in optical fibers (or other waveguiding media) which are isotropic, and therefore suitable for these applications.